Needle coke is generally produced by using heavy oil such as petroleum heavy oil or coal tar as a raw material, and is used as an aggregate of a graphite electrode for electric steelmaking. In the process of producing a graphite electrode, needle coke having a predetermined grain size is first mixed with a binder pitch in a predetermined ratio, and then the obtained mixture is extrusion-molded, sintered, and graphitized.
The graphitization is performed by a heat treatment at about 3000° C., and a method using a direct-passage-of-electric-current type furnace (LWG furnace) is generally employed for the graphitization. In this method, since the temperature rises quickly, gas is generated at high speed due to an impurity such as sulfur or nitrogen contained in coke, so that irreversible expansion called “puffing” occurs. The occurrence of puffing leads not only to a decrease in electrode density but also to a fracture in the electrode.
In addition, since a graphite electrode is used under severe conditions such as a high-temperature atmosphere, a low coefficient of thermal expansion (CTE) is needed. That is, as the CTE decreases, electrode wear in the process of electric steelmaking decreases so that costs for electric steelmaking can be reduced. To reduce the CTE of a graphite electrode, the CTE of needle coke needs to be reduced.
Various methods for producing needle coke have been proposed for reducing puffing and CTEs in the needle coke. Patent Document 1 describes that bottom oil of a residue fluid catalytic cracking (RFCC) apparatus and vacuum residual oil are mixed, and then subjected to delayed coking. Patent Document 2 describes that first heavy oil obtained by hydrodesulfurization of heavy oil under a total pressure of 16 MPa or more and second heavy oil derived from a residue fluid catalytic cracking (RFCC) apparatus are mixed, and then subjected to delayed coking. Patent Document 3 describes that first heavy oil obtained as vacuum residual oil and second heavy oil derived from a residue fluid catalytic cracking (RFCC) apparatus are mixed, and then subjected to delayed coking.